Traditional means of isolating selected gases from a mixed stream involve physical, chemical reactions or a combination thereof, and inert semipermeable membranes. Among such processes are cryogenic, gas-liquid and gas-solid sorptive techniques (e.g. pressure swing adsorption, amine treatment, iron sponge, etc.), and immiscible liquid--liquid extraction (for recent summaries see Michaels A S: New vistas for membrane technology, Chemtech. 19:160-172, 1989; and Babcock R E, Spillman R W, Goodin C S & Cooley T E: Natural gas cleanup: A comparison of membrane and amine treatment processes. Energy Prog. 8(3):135-142, 1988.) Newer technologies focus on the use of inert semipermeable membranes but these do not offer a separation solution that is particularly unique over existing processes (Spillman R W: Economics of gas separation membranes. Chem. Engr. Prog. 85:41-62, 1989). Membrane systems have been said never to achieve complete separation (Spillman, id. 1989). Prior art physical or chemical means do not readily allow segregation among gases with similar physical or chemical properties or those in low concentrations. In general prior art does not effectively deal with extracting gases or gas equivalents from a dissolved or ionized state to regenerate a purified gas. The prior art generally treats gases already dissolved in water such as carbon dioxide or oxygen in Bonventura et al., U.S. Pat. Nos. 4,761,209 and 4,602,987 and carbon dioxide in Henley and Chang U.S. Pat. No. 3,910,780. No reference has been located in which the enzyme contacts a gas in a gas stream, separates the gas and in a subsequent step regenerates a purified gas.
Traditional gas separation means commonly exhibit one or more of the following problems: they are energy inefficient, commonly nonspecific, quite slow, require a relatively pure feedstock, depend on a significant pressure head, or use ecologically questionable or toxic compounds. The relatively pure feed stock requirement may result in a geographical restriction of available feed materials. The geographic availability may require shipment from distant locations such that transportation costs may be high, and even prohibitive for some uses. The preceding limitations present restrictions on the growth and application of gas extraction/purification systems. A gas separation or enrichment process that did not require highly concentrated feed-stocks thus eliminating or reducing transportation requirements would be beneficial.
In contrast to the disadvantages enumerated above for traditional physical/chemical methods, biological catalysts (enzymes) present several advantages including enhanced efficiency, speed, and increased specificity. Enzymes also commonly distinguish optical isomers. Further, they can be used at moderate temperatures and pressures, enhancing safety.
Prior use of enzymes has focused very largely on the food processing industry, cleansing or detergent applications, or processing of sewerage. Industrial applications in the gas field have been limited. Prior application of enzymes to gas extraction are found in patents to Bonaventura et al, U.S. Pat. Nos. 4,761,209 and 4,602,987 and Henley and Chang U.S. Pat. No. 3,910,780. Bonaventura uses membranes impregnated with carbonic anhydrase to facilitate transport of CO.sub.2 across a membrane into water in an underwater rebreathing apparatus. Henley and Chang make a similar use carbonic anhydrase. Both processes operate on dissolved carbon dioxide. Neither taught fixation of the enzyme with the active site exposed to the gaseous phase with sufficient hydration to maintain a reactive conformation. Neither taught modification of DNA coding for enzymes to build in specific structure for fixation or enhanced catalysis. Indeed Bonaventura took for granted that the crude coupling techniques disclosed would deactivate a large fraction of the active enzyme. The Bonaventura patents contain computations showing that only a small fraction (1%) of the carbonic anhydrase need retain its activity in the bonded membrane to provide adequate capacity to remove carbon dioxide from the proposed apparatus in the illustrative uses. Henley and Chang do not discuss activity losses nor provide any description of fixation techniques to enhance enzyme activity when in the active site is directly exposed to a nonaqueous environment.
Despite some significant advantages, a variety of major problems have limited the application of enzymes in industrial settings. These include short lifetime of either free or immobilized enzyme, fouling and biofouling, separation of the enzyme from the immobilization surface, limited availability of enzymes in sufficient quantity, and expense of manufacture.
These problems have resulted in relatively few efforts to use enzymes for manipulation of gases. Further, physical/chemical means are in place commercially; they are understood and represent established technology and significant investment.
Despite these historic considerations a number of recent developments now allow broad based enzymatic applications. First, the development of DNA libraries and the techniques needed to generate such libraries so that large amounts of enzyme can be made economically. Previously, and even today, many enzymes are derived by purification from a biological source. Second, development of techniques to generate membrane expression of enzymes and even direct secretion such that harvesting the enzymes is easier and economically feasible. Third, the development of new immobilization techniques which allow long lifetime and high efficiency.